Cathode ray tubes (CRT's), similar to those used to display television images, are commonly used to display both text and graphics information derived from such sources as computers, videotext sources, computer data bases and the like. Such systems offer a cost effective and high-resolution means for displaying information. The information displayed in such applications is usually derived from a digital signal representing the information to be displayed on the CRT. For example, a logic "1" bit commonly corresponds to a bright dot and a logic "0" bit will produce no reaction from the screen. This contrasts with the use of CRTs in a classical television system wherein the signal representing the displayed information is derived from an analog signal source. Nevertheless, the signal that ultimately drives the CRT in both applications is an analog one.
The image drawn on the CRT display is produced by an electron beam striking the back of a transparent glass screen coated with a flourescent material that emits light in response to being excited by the electron beam. The intensity of the displayed image is controlled by the intensity of the electron beam striking the flourescent screen.
The image displayed on a conventional raster scan CRT is made up of a series of frames each lasting about 1/30th of a second. Each frame is commonly made up of two fields lasting about 1/60th of a second. The fields are made up of a number of scan lines (typically 262.5, the NTSC standard employed in the United States for commercial television). The display is further adapted to interlace the scan line of each field so that a complete frame comprising two fields is made up of 525 scan lines. The number of scan lines determines the vertical resolution of the display.
Although the foregoing description applies to a typical NTSC television display, the same principles are generally applicable to a CRT display used in a computer application. In such a system the number of scan lines and the interlacing process may be modified, but the general principal of sequential display of scan lines comprising successive frames remains the same. The horizontal display resolution of the CRT is a function of the speed at which the electron beam scanning the screen can be modulated by the system.
A common way of displaying text information on a CRT is to define each text symbol as comprising a grid of display dots. Typically, modern systems define a text character within a 5.times.7, 7.times.9, or 12.times.16 dot matrix or character cell. When a character is to be displayed on a CRT, the dots representing the text symbol in the vertical direction are represented on successive horizontal scan lines. In the horizontal direction, the dots are displayed by turning the electron beam on and off as it traverses the screen. For example, if a text or graphical symbol is represented in a cell comprising 5.times.7dots there will be thirty-five possible dots that may or may not be illuminated according to the appropriate symbol to be displayed. If, for example, the symbol T is to be displayed, the first scan line at the location at which the T is to be shown on the CRT will be active for a duration corresponding to five dots, thereby displaying the top portion of the T. During the next successive six scan lines the beam will only be active for a duration corresponding to one dot location thereby drawing out the vertical portion, or "stem" of the T.
The fact that the vertical portion of the T is made up of single dots standing alone in the video stream often leads to an apparent intensity imbalance, as discussed in more detail hereinafter. There are considerable demands made on a CRT display system when it is required to display a number of symbols per line. These demands can best be illustrated by considering the requirement of a typical computer terminal used to display either text or graphics symbols. As before, assume each symbol consists of a matrix of 5.times.7dots. A typical CRT system is required to display eighty such characters per line (in the previous sentence, a "line" refers to a line of character which is comprised of, in the example, 7 horizontal scan lines of the beam). Eighty characters will require the possibility of at least 400 (80.times.5) dot locations in the horizontal direction. In order for the CRT to accurately display such dots it is necessary for it to be capable of turning on and off more than 400 times per scan line, because there are extra bits or "backfill" between characters.
In a typical display system each scan line may take roughly 63 microseconds, of which about 55 microseconds are available for displaying information. The extra 8 microseconds are required to allow the beam to return to its starting position on the next line. For a beam to be capable of turning on and off 400 times in 55 microseconds, a video monitor bandwidth of about 7 megahertz is required. That is, the beam must be capable of turning on and off 7 million times per second. The horizontal bandwidth of a typical commercial television is only about 3.8 MHz, making it unsuitable for high information density video applications.
When a video monitor lacks the required bandwidth, the resolution and hence the clarity of the display will be compromised. Returning to the example of the "T", the top portion will be relatively bright even if the video monitor's bandwidth is relatively low. This is so because the beam drawing the top portion remains on as it traverses the character cell. The response time of the video monitor is thus not particularly critical. However, during successive scan lines, when the "stem", or lower portion, of the T is to be drawn, the beam is only on for 1 dot period. Therefore, if the video monitor lacks sufficient bandwidth, the stem will be only dimly displayed, if displayed at all. This leads to an apparent intensity imbalance between portions of the same character and between different characters on the screen.
The intensity imbalance problem can be alleviated using video monitors with sufficient bandwith, for instance a video monitor with a 12 MHz bandwidth would be suitable in many applications. As the resolution required increases, as would be the case with more characters or higher dot density per character cell, the bandwidth required also increases. Video monitors with high bandwidths and short rise times are very expensive and are therefore not a practical solution in many cases. For example, a video monitor with a 15 MHz bandwidth and a video amplifier rise and fall time of 20 ns costs roughly $100, whereas a video monitor with an 80 MHz bandwidth and a video amplifier with a 4.5 ns video amplifier rise and fall time may cost in excess of $1,000.
It is accordingly an object of the invention to provide a cost effective means to balance the apparent intensity between horizontal and vertical line segments on a video screen.
Another object of the invention is to provide for selective intensity balancing of a video display.
A further object of the invention is to provide for video intensity balancing in both normal and reverse video modes of operation.
Still further objects and advantages of the invention not specifically enumerated here will become readily apparent upon consideration of the following drawing, description and claims.